There are several types of step-regulating AC line regulators available from several manufacturers. Among these are tap-switching and multi-primary switching devices. These two types have several advantages because they combine many desirable features such as low cost, small size, no generated noise, and high efficiencies in the order of 99 percent.
The present and future demand for electrical power is increasing to such a level that the power companies can no longer satisfy the demand, and thus power shortages and brown-outs occur frequently. At the same time, electronic equipment, computers and communications systems have become so sophisticated that such equipments can only operate with well regulated AC input power. So variation in line voltages to such equipments, as result from over-load conditions, power shortages, brown-outs or just major changes in load conditions can adversely affect such equipments, which will have catastrophic results in critical applications. One solution to these problems which is being offered for those critical applications is known as Uninterruptable Power Systems. These systems have an energy storage; usually a bank of batteries that are constantly being charged from the AC input power. When power interruptions occur the storage devices then supply DC power to a converter which generates regulated AC power for the critical system. The uninterruptable power systems are not only bulky but they are very complex and expensive. Step-switching regulators can be used for such critical applications if their speed of response is made fast enough, and if the regulation steps do not cause unsummetric output waveforms. Unsummetric output waveforms are equivalent to a DC component in an AC circuit and can cause saturation in the magnetic components of the load. Of course, those magnetic components are the power transformers in the critical systems.
High performance, step-switching regulators make their ranging steps at zero voltage crossing so that no switching noise is generated. Once a range step has been made, no further correction can be made until the next zero crossing. However, if range steps are permitted to occur in successive half cycle intervals, then this generally causes unbalanced output voltages. These output voltages are substantially the same as or comparable or equivalent to the unsymmetric output waveforms. A practical solution to this problem is to limit the range stepping so that it occurs only once during a full cycle, i.e. on every second zero crossing. But this approach represents a severe limitation on the speed of response time and makes the step-switching regulator useless for all critical applications. It is therefore desirable to have a step-switching regulator that can respond to line voltage transients in successive half cycle intervals, without generating any magnetic unbalance in the output.